refactor(mcp-server): make the diagram modules usable from the web app

The web app will reuse the MCP server's XML engine instead of its own
copy in lib/utils.ts, so these modules now run in the browser too.

- Relative imports end in .ts, rewritten to .js by tsc
  (rewriteRelativeImportExtensions); Next.js resolves them directly
- Every module uses the global DOMParser/XMLSerializer: native in the
  browser, linkedom in Node via installDomPolyfill. pages.ts parsed with
  linkedom but serialized with the global serializer, which throws in
  the browser
- The saxes syntax check moves to xml-syntax.ts, so the browser does not
  pull in linkedom; it now also rejects undeclared prefixes such as
  xlink:, as the browser does
- Page decompression uses pako and atob instead of node:zlib and Buffer
- hasCells moves to pages.ts, away from the file system code
- The duplicate cell id check counts UserObject/object ids
- wrapCellsInModel drops comments and text before the first cell, which
  the web app accepts today
- validateAndFixXml takes { strict: false } for diagrams with user content
- Web tests run these modules with a browser DOM (jsdom)
- saxes becomes a direct dependency of the web app

packages/mcp-server/package.json

@@ -42,11 +42,13 @@
         "@modelcontextprotocol/sdk": "^1.31.0",
         "linkedom": "^0.18.0",
         "open": "^11.0.0",
+        "pako": "^2.2.0",
         "saxes": "^6.0.0",
         "zod": "^4.0.0"
     },
     "devDependencies": {
         "@types/node": "^24.0.0",
+        "@types/pako": "^2.0.4",
         "tsx": "^4.19.0",
         "typescript": "^5",
         "vitest": "^4.1.8"